augment USE_GCC description in the porter's handbook

[Alexey Dokuchaev]

hi,

cooked something up, so people would get less confused about proper
USE_GCC usage.  diff attached.

./danfe
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Index: book.xml
===================================================================
--- book.xml	(revision 42655)
+++ book.xml	(working copy)
@@ -3822,10 +3822,11 @@
 	    <filename>/etc/make.conf</filename>.  For instance,
 	    setting</para>
 
-	  <programlisting>USE_GCC=3.4</programlisting>
+	  <programlisting>USE_GCC=X.Y</programlisting>
 
-	  <para>would add a dependency on gcc34 for every port,
-	    including gcc34 itself!</para>
+	  <para>(where X.Y is version number) would add a dependency on
+	    gccXY for every port, including <literal>lang/gccXY</literal>
+	    itself!</para>
 	</note>
 
 	<table frame="none">
@@ -3856,17 +3857,23 @@
 
 	      <row>
 		<entry><makevar>USE_GCC</makevar></entry>
-		<entry>The port requires a specific version of
-		  <command>gcc</command> to build.  The exact version
-		  can be specified with value such as
-		  <literal>3.4</literal>.  The minimal required
-		  version can be specified as <literal>3.4+</literal>.
-		  The <command>gcc</command> from the base system is
-		  used when it satisfies the requested version,
-		  otherwise an appropriate <command>gcc</command> is
-		  compiled from ports and the <makevar>CC</makevar>
-		  and <makevar>CXX</makevar> variables are
-		  adjusted.</entry>
+		<entry>The port requires GCC (<command>gcc</command> or
+		  <command>g++</command>) to build.  Some ports need any
+		  GCC version, some require modern, recent versions.  It
+		  is typically set to <literal>any</literal> (in this
+		  case, GCC from base would be used on versions of &os;
+		  that still have it, or <literal>lang/gcc</literal> port
+		  would be installed when default C/C++ compiler is Clang);
+		  or <literal>yes</literal> (means always use stable, modern
+		  GCC from <literal>lang/gcc</literal> port).  The exact
+		  version can be also specified, with a value such as
+		  <literal>4.7</literal>.  The minimal required
+		  version can be specified as <literal>4.6+</literal>.
+		  The GCC from the base system is used when it satisfies
+		  the requested version, otherwise an appropriate compiler
+		  in built from the port, and the <makevar>CC</makevar>
+		  and <makevar>CXX</makevar> variables are adjusted
+		  accordingly.</entry>
 	      </row>
 	    </tbody>
 	  </tgroup>